The present invention relates to manufacture of micromechanical structures, and relates in particular to a method for narrowing a gap between micromachined structures on a device during manufacture in an epitaxial reactor.
A method of depositing structural layers during manufacture of surface-micromachined devices sometimes involves the use of an epitaxial reactor. Epitaxy is a process for production of layers of monocrystalline layers of silicon over a single crystal substrate, and for forming polycrystalline silicon layers over other substrate materials, for instance SiO2 films on silicon substrates. Epitaxial reactors may be operated with precisely controlled temperature and environmental conditions to ensure uniform deposition and chemical composition of the layer(s) being deposited on the target substrate. In addition to the precise control, use of an epitaxial reactor may permit build-up of layers on a substrate at significantly higher rates than typically found with LPCVD (Low Pressure Chemical Vapor Deposition) systems.
U.S. Pat. No. 6,318,175 discusses an approach to using epitaxial deposition to create a micromachined device such as a rotation sensor.
While the foregoing micromachining operations or similar processes may provide acceptable products for many applications, some applications may require finer width gaps between the micromachined elements on the device than can be provided by this process. Some applications may require, for instance, obtaining higher working capacitances and/or electrostatic forces between micromachined structures. While etching very narrow trenches to obtain desired narrow gaps has been attempted, these methods may require slower etch rates, may be limited in aspect ratio, and may be subject to limitations of the lithography and etching process. Similarly, germanium has been applied to produce narrow gaps, however, this process may have process compatibility limitations.
Accordingly, there is a need for a process for manufacturing devices which provides product with inter-element gaps that may be precisely defined or xe2x80x9ctunedxe2x80x9d to meet the device design objectives, while still maintaining satisfactory production rates.
According to an exemplary embodiment of the present invention, a method for precisely controlling the gap between micromechanical elements on a device, or xe2x80x9cgap tuning,xe2x80x9d begins with a partially formed micromechanical device, which may comprise a substrate layer of, for example monocrystalline silicon or a SiGe mixture. A sacrificial layer of, for example, SiO2 may be deposited on the substrate layer. A functional layer of, for example, epitaxially deposited silicon, may be etched after application to define micromechanical structures or devices thereon.
Once the elements of the micromechanical structure or device have been defined in the function layer and the sacrificial layer, in situ cleaning of the device within the epitaxial reactor may be performed. The cleaning may be performed, for example, with hydrogen (H2) to remove surface oxides, and/or with hydrochloric acid (HCI) to remove silicon residues and surface imperfections resulting from the trench etching process. Following the cleaning step, gap tuning may be performed by selectively depositing an epitaxially-grown layer of silicon on the surface of the partially completed device, and in particular on the sides of the previously etched trenches defining the micromechanical elements in the function layer. As the gap-tuning layer is deposited, the gap width may be monitored, for example with an optical end-point detection system. The gap tuning deposition may be halted when the inter-element gap has been narrowed to the desired extent.
The precision control of the width of the gaps between the micromechanical elements on a device in the foregoing manner may provide several advantages including: ready compatibility with an epitaxial environment and standard epitaxy equipment; high production rates due to the high layer deposition rates that may be achieved in an epitaxial reactor; and ready adaptability to use of different materials to be deposited on the micromachined device, including monocrystalline silicon, polycrystalline silicon, a SiGe mixture, pure germanium, or silicon carbide. Furthermore, the deposited layers may be in situ doped.